Electrically shielding casings for accommodating and electromagnetically shielding units which emit electromagnetic radiation or which are sensitive to electromagnetic radiation have long been known.
While earlier such casings were usually made from metal for obvious reasons, with the onset of the spread thereof on a large scale--for example in relation to mobile telephones or cordless telephones--for cost and weight reasons there has been a change over to making them in particular from plastic material. The prefabricated, in particular injection-moulded casing portions are coated with a conductive material, for example by spraying on conductive lacquer, vapour deposition with aluminium or galvanisation.
Such casings were initially generally provided with prefabricated seals which comprise conductive elastomer and which were fitted upon assembly. DE 38 12 943 A1 shows an internally coated shielding casing of fibre-reinforced plastic material with a prefabricated seal of that kind which was inserted between a groove and a tongue.
Casings of the above-indicated kind are also known from EP 0 629 114 B1and EP 0 654 962 A1.
The casings described therein are composed of two portions which for electrical shielding of the interior of the casing (at least in portion-wise manner) comprise elastically conductive material or are coated with same and in the assembled condition form a Faraday's cage For the purposes of electromagnetic shielding of the casing in the join region between the casing portions which come together, there is provided a shielding seal which comprises an electrically conductive and at the same time elastic material and which adapts to surface tolerances and irregularities so that a very high level of quality in terms of shielding the interior of the casing can be guaranteed even on a mass production scale. This shielding seal is produced directly on at least one of the casing portions while an additional carrier can also be incorporated into the structure.
By virtue of that configuration the casing can be opened for example for maintenance purposes or to replace a power source and then easily closed again while maintaining the sealing and shielding effect.
In production of a material for the shielding seal however it was found that it was difficult to arrive at an optimum compromise between high elasticity, high conductivity and lowest possible material costs. A material with an advantageously high shielding action is a plastic material which is filled with a high proportion of silver powder but this is relatively expensive and has only limitedly satisfactory mechanical properties DE39 34 845 A1 describes the production of a shielding casing with a two-layer shielding profile by the application of an elastic but non-conductive sealing profile to a casing portion and then spraying a conductive coating on to that sealing portion. This method basically makes it possible to save on material costs but it is technologically complicated and expensive and results in seals whose mechanical and in particular electromagnetic properties cannot be completely convincing.
The solution proposed in WO 97/26782 by the applicants also requires if the sealing profile is dispensed on to a casing portion--an additional method step and only makes it possible to produce relatively thin conductive coatings.